Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP LTE systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out (SISO) system, multiple-in-signal-out (MISO) system, or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports a time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point (AP) to extract transmit beam forming gain on the forward link when multiple antennas are available at the access point (AP).
In such systems, the signal at a receiver, such as the receiver of a wireless access terminal (AT), may vary substantially in power level due to a number of factors, including transmit power, mobility, multipath, fading, interference and others. At such receiver, the received signal is typically amplified by a low noise amplifier (LNA), down-converted to a lower frequency range by a mixer-local oscillator (LO) combination, and converted into digital format by an analog-to-digital converter (ADC). In order to properly or optimally convert the signal into digital format, the average signal power level at the input of the ADC should be large enough that the quantization-to-noise ratio is sufficiently large to properly quantized the received signal with minimal noise, and small enough to prevent saturation of the ADC. Thus, in order to properly set the signal power level at the input of the ADC, typically such receivers employ automatic gain control (AGC) to adjust the gain of a device upstream from the ADC.
Another issue that arises in such receivers is the DC level produced at the output of the downconverting mixer and other devices downstream thereof. The DC level is mainly produced by the LO signal leaking towards the input, and then re-mixing with the LO signal again to produce the DC level. Generally, the DC level adversely affects the estimation of the received signal power level performed downstream. This estimation is performed to properly set the gain of the LNA as well as a digital gain stage further downstream.
Generally, the AGC of the LNA or mixer gain and the DC level are not independent of each other. For example, changing the gain of the LNA or mixer often causes the DC level to change as well. Thus, there is a need for a technique to perform AGC and DC level compensation in a combined fashion to ensure proper signal level at the input of the ADC, as well as a reduction or elimination of the DC level downstream of the mixer.